1. Field of the Invention
The present invention relates to a specimen image display apparatus used with a scanning electron microscope, an X-ray micro-analyzer or a similar device.
2. DESCRIPTION OF THE PRIOR ART
In a scanning electron microscope, specimen signals including secondary electrons, reflected electrons, absorbed electrons or transmitted electrons generated by the scanning of an electron beam on the surface of a specimen are detected, and the resulting detection signal is used to display a magnified image of the specimen on a cathode-ray tube of the specimen image display apparatus. The magnification of the specimen image on the cathode-ray tube is expressed by the ratio between the scanning width of the electron beam scanning the surface of the specimen and the scanning width of the beam on the cathode-ray tube for displaying the specimen image. For instance, let the horizontal scanning width of the beam on the cathode-ray tube be x.sub.1 and the horizontal scanning width of the electron beam on the surface of the specimen be x.sub.2. Then the magnification M of the specimen image on the cathode ray tube is given as M=x.sub.1 /x.sub.2.
In observing or photographing the specimen image displayed on the cathode-ray tube, the length of the specimen image is measured. Conventional scanning electron microscopes are so constructed that for the purpose of measuring the length of the specimen image, a scale used for measuring the length of the specimen is displayed in addition to the specimen image on the cathode-ray tube. This method of display is disclosed in Japanese Patent Laid-Open No. 47262/78. The length of this scale changes with the degree of magnification of the specimen image on the cathode-ray tube, and therefore it is possible to measure the length of the specimen image on the cathode-ray tube using the scale displayed on the cathode ray tube as a reference.
In view of the fact that the scale on the cathode ray tube changes with the magnification of the specimen image as mentioned above, however, it is difficult to measure the length of the specimen image accurately in the range of magnification where the scale is displayed in short form.